1. Field of the Invention
The invention relates to a skeletal prosthetic implant containing a bonded porous fixation structure on its surface. More particularly, the porous ingrowth structure is comprised of at least one slotted plate diffusion bonded to the surface of a metal prosthetic implant or bonded in a recess formed on the surface.
2. Description of the Prior Art
Tissue ingrowth surfaces intended to improve fixation of prosthetic implants have experienced increasing acceptance in the orthopedic field in recent years. In the past, most implants were fixed using a polymethyl methacrylate bone cement to achieve prosthesis fixation. However, recent experience has shown that fixation utilizing tissue ingrowth into porous coated implants has achieved success rates equivalent to prostheses fixed with cement.
In most cases, the porous structures or coatings utilized to create fixation by tissue ingrowth were loosely packed sintered metal powders, kinked pressed metal fibers, woven metallic meshes, or expanded metal sheets as well as porous polymeric materials.
Examples of tissue ingrowth surfaces in the form of meshes are shown in U.S. Pat. Nos. 3,905,777, 3,938,198, 4,089,071, 4,261,063, 4,536,894, 4,636,219, 4,644,942, 4,813,959, 4,813,960, 4,863,474 and 4,863,475.
Examples of metallic particles bonded to the surface of orthopedic implants to encourage tissue ingrowth are shown in U.S. Pat. Nos. 3,605,123, 4,542,539, 4,550,448 and 3,855,638. U.S. Pat. No. 4,599,085 relates to an implant member comprising sintered metal plus bioactive ceramics which encourage tissue ingrowth.
U.S. Pat. No. 4,660,755 relates to the use of resistance welding to bond meshes to a substrate.
U.S. Pat. No. 4,854,496, relates to a porous metal coated implant where spherical particles are diffusion bonded to an implant made from titanium.
Each of these structures has its own characteristic porosity, which is a function of the materials and processes used to create the structure. While porous structures may vary from coating to coating, within a given porous coating, the structural porosity is normally constant, and can be defined in terms of pore size, pore size distribution and overall pore volume. It is difficult to vary these structures to form a wide variety of pore sizes and pore size distributions.
Even in clinically successful uses of the prior art tissue ingrowth structures, the ingrowth of biological tissue is found to be somewhat sporadic in a variable composition and comprised of substantial amounts of soft connective tissues with only partial proportions of bone. Fixation is enhanced where larger amounts of bone or hard connecting tissue grows into the prosthesis surface rather than merely soft connective tissue. Laboratory histological examination of retrieved clinical human implants and experiments conducted in animals have shown that there is a relationship between the pore size and ingrown bone quality. It has been found that fine pores encourage soft connective tissue ingrowth while larger pores favor hard or bone tissue ingrowth. It has also been found that hard or dense cortical bone exhibits a faster ingrowth rate than spongy cancellous bone. In general, tissue ingrowth develops a preferential orientation in response to the direction of loading applied across the implant-bone interface.
The present invention utilizes these relationships between ingrowth tissue quality, coating pore size and the relationship of the loads applied between the prosthesis and bone interface to produce a porous ingrowth surface which can be easily tailored to take advantage of these known design parameters. The present invention, therefore, is in contrast with existing coatings. The prior art coatings offer no provision for tailoring the porous coating as necessary to address the variability of bone at the surgical sight or to achieve a preferred tissue orientation to resist anticipated in-service loading.